Most of the times if you enabled MFA (Multi Factor Authentication aka. 2 Step verification) your organizational tenant users wont be able to authenticate into applications with typical password they use.
Applications could be desktop or mobile applications eg. Outlook, ios email, Android email etc.
As an admin this will be some over work and for Office 365 users this will be little bit of confusing. To overcome confusions following simple steps will be useful.
Login into Office.com
Copy and paste below address on same browser.
Generate app password and enter that password to your app instead of usual password.